The load estimation/load prediction functionality of the enhanced uplink (EUL) scheduling function is needed to assess the predicted uplink air interface load, given a tentative scheduling decision. In the Wideband Code Division Multiple Access (WCDMA) cellular system users can be added to the EUL channel up to the point where the uplink load reaches the scheduling threshold. This scheduling threshold is determined from two requirements: the load allowed, so as not to impair the planned coverage and the load allowed, so as not to impair cell stability.
The load affecting the coverage can be obtained as the Rise over Thermal (RoT). This measure is obtained by measuring the total received WCDMA wideband power and dividing this with the thermal noise floor. The total wideband power consists of the noise floor power plus neighbour cell interference plus normal power controlled traffic in the cell, plus the scheduled enhanced uplink traffic. The load allowed, so as not to impair cell stability, does not include the neighbour cell interference—simply because that power is not controlled by the present cell and that power can therefore not cause instability, at least not as a first approximation.
It may be distinguished between the situation where the load is measured (“now”) and predicted (“evaluation of tentative EUL scheduling decisions”). The present solution relates to the latter case, with load prediction.
In case of load prediction, e.g. for stability, the predicted load factor is computed from the measured Signal to Interference Ratio (SIR) on the control channel, and from power factors (beta factors) that express and add the data channel power to the control power, to arrive at a total predicted load power for a user. These predicted powers are then summed up as load factors, and compared to the thresholds to see if the tentative scheduling decision is feasible or not.
The WCDMA enhanced uplink aims at scheduling traffic to times when the uplink interference situation is favourable, thereby utilizing air interface resources in a better way than before. The air interface load is measured by the noise rise, over the thermal level, a quantity denoted rise over thermal (RoT). This idea is illustrated in FIG. 1.
FIG. 1 illustrates the air interface load according to prior art. The pole capacity is the limiting theoretical bit rate of the uplink, corresponding to an infinite noise rise.
The noise rise may be seen as the total received power relative to the noise power within a cell. The noise rise is increasing with the number of user equipments and/or radio traffic intensity within the cell.
The uplink data channel is denoted Enhanced Dedicated Channels (E-DCH) Dedicated Physical Data Control Channel (E-DPDCH). This channel supports a high rate. It is however not involved in the scheduling control as such, this is the task of the corresponding control channel, denoted Enhanced Dedicated Channels (E-DCH) Dedicated Physical Control Channel E-DPCCH. This channel e.g. carries rate requests (measurement signals) from the user equipments to the EUL scheduler, situated at the base station. There are also some downlink channels supporting EUL. The first of these is the Enhanced Dedicated Channels (E-DCH) Absolute Grant Channel (E-AGCH) channel which carries absolute grants (control signals) to each user equipment. More peripheral is the E-DCH Relative Grant Channel (E-RGCH) channel which carries relative grants (also control signals) from the base station to the user equipment. Finally, the E-DCH Hybrid automatic repeat request (HARQ) Acknowledgement Indicator Channel (E-HICH) channel carries acknowledgment/non-acknowledgment (ACK/NACK) information and is not directly involved in the present solution.
The grants mentioned above are the quantities signalled to the user equipment indicating what rate (actually power) it may use for its transmission. The user equipment can, but need not, use its complete grant. Relative grants are used to control the interference in neighbour cells. These can only decrease the current grant of the user equipment one step. It is stressed that there are only a discrete number of grant levels that can be used.
The task of the scheduler is to schedule EUL user traffic, to enhance user and cell capacity, at the same time as it keeps track of the air interface cell load, avoiding over-scheduling that may cause cell instability and loss of coverage. Also, the scheduler keeps track of other available traffic, like transport resources and hardware. Further, the scheduler receives, measures and estimates quantities relevant for its scheduling operation. In addition, the scheduler also transmits orders to user equipments, primarily in the form of granted power/bitrates.
The present solution mainly relates to the scheduling of EUL user traffic, to enhance user and cell capacity, at the same time as it keeps track of the air interface cell load, avoiding over-scheduling. In particular to the load prediction step needed to evaluate tentative scheduling decisions.
US 2004/0252666 A1 discuss a method for managing uplink radio resources in a CDMA communication system, based on determining the interference level into the primary base station, determining a contribution of secondary cell connections to the interference level and computing a proportionality factor for adjusting a reference interference level relative to the interference level.
A problem when performing load prediction is due to the fact that the SIR measurement in WCDMA is noisy. This causes the predicted effect of the tentative scheduling decision to vary quite a lot between adjacent sampling times. This in turn affects the scheduler in a negative way, preventing an optimal use of available uplink air interface resources.
A further problem that may occur under certain conditions is that when the user equipment is in soft(er) handover, conflicting power control commands may result since both base stations try to control the transmit power of the user equipment.
Soft handover refers to a feature used by e.g. the CDMA and WCDMA standards, where a user equipment is simultaneously connected to two or more cells, or cell sectors, during a call. If the sectors are from the same physical cell site, i.e. a sectorised site, it may be referred to as softer handover. This technique is a form of user equipment-assisted handover, for user equipments which continuously make power measurements of a list of neighbouring cell sites. It is then determined whether or not to request or end soft handover with the cell sectors on the list.
As a result the mean value of the measured SIR and the commanded SIR target may start to drift apart in the serving cell. In such a situation it will be required to resort to the measured SIR for load estimation. The problem is that it is non-trivial to determine when a statistically significant difference exists between the target SIR and the measured SIR, when in soft(er) handover—the reason for this being the very noisy SIR measurement.